Springs are often used in the chassis of motor vehicles. They are consequently likewise subject to the demands made in striving for lightweight construction, which as the very first consideration are directed at these unsprung masses. In this connection, the use of fiber composite materials (FCM) has already been proposed many times. This applies in particular to torsion springs of carbon fiber reinforced plastics (CRP) or glass fiber reinforced plastics (GRP). It is particularly problematic here to produce these components appropriately for the loading concerned but at the same time at low cost.
A further difficulty is that the fibers can in each case only transfer tensile forces or compressive forces, and consequently the macroscopic shear loading in the spring wire has to be divided into a tensile component and a compression component (with respect to the principal axes of stress, +−45° to the bar axis in accordance with Mohr's theory of stress).
FCM springs that are known today are aimed at accommodating the tensile and compressive force distribution in the material as favorably as possible by means of windings of the fibers at an angle of +/−45° to the bar axis.
Also known are exclusively +45° tensile fiber windings, the shear stress components being borne by the matrix material or by compressive stresses in the core.
The aim in the structural design of suitable springs is for the entire spring material that is used to be utilized homogeneously in terms of loading. It is thus intended that there should not be any dedicated weak points in the material, but rather that the entire material should reach its loading limit under uniform maximum loading. This corresponds to the best possible utilization of the material, and consequently to the best achievable degree of lightweight construction.
EP 0637700 describes a spring construction in which carbon fibers that are wound at an angle of +−30° to +−60° around the bar axis are used. A characterizing feature is that the number of tensile fibers used differs from the number of compression fibers. In particular, the number of compression fibers is increased in comparison with the tensile fibers. The aim of this is a more uniform loading of the fibers, which brings about a better specific utilization of the material used. Although the material is better utilized as a result of the fibers of the tensile direction and the compressive direction being used in different quantitative ratios, or different layer thicknesses, the dependence of the material utilization on the diameter of the spring wire is not eliminated.
U.S. Pat. No. 5,603,490 proposes only using fibers in the tensile direction and no compression-loaded fibers. The fibers are to be wound up in such a way that they are only tensile-loaded. In the case of a spring with a hollow-profile cross section, this would quickly lead to failure due to the shear stresses, for which reason a compressively stable core that absorbs the stresses is required here. However, the constant state of hydrostatic stress in the core and the state of shear stress in the wound fiber envelope lead to a disadvantageous creep of the plastics matrix (epoxy). Therefore, this solution cannot be used for example for applications in vehicle construction (continuous loading due to the weight of the vehicle). Although the use of only one direction for the fibers means that the tensile loading potential of the fibers is optimally used, shear stresses that then have to be transferred for the most part through the plastics matrix due to the lack of compressive fiber support mean that strong creep effects occur under continuous loading.
WO 2014/014481 A1 proposes a fiber construction in which the number of fibers in the layers and the core are multiples of a common base number. The use of a number of different materials in a spring (for example glass, carbon or a mixture) is also disclosed. In addition, it is disclosed that the angles of the individual fibers of the fiber plies in relation to the bar axis can alternate (in particular between a positive angle and a negative angle). The core of the spring may consist of unidirectional fibers, but a solid core or a hollow core is also disclosed. A core of a material with a shape memory is also proposed. Although it is mentioned that the spring material may be composed of mixed materials, no instruction is given, as a result of which the procedure and effect of a mixed type of construction remain unclear. The fibers should be arranged in the layers in a number that is an integral multiple of a common reference base, the effect likewise remaining unclear. This arrangement has the disadvantage that the fibers are only present in the layers in numbers of integral factors, and consequently an optimum layer thickness adaptation is not provided.
The types of spring construction from the prior art do not achieve an optimum degree of lightweight construction, since they do not effectively utilize the material that is used.
Table 1 shows exemplary embodiment 1 of the design method with a braided textile and a core diameter of 4 mm.
Table 2 shows the fiber materials used for exemplary embodiment 1 with their properties, the properties which are known from the prior art and have merely been compiled here.
Table 3 shows exemplary embodiment 2 of the design method with a wound textile such as on a coiling machine, for example, and a core diameter of 3.5 mm.
Table 4 shows the fiber materials used for exemplary embodiment 2 with their properties, the properties which are known from the prior art and have merely been compiled here.
Table 5 shows exemplary embodiment 3 of the design method with a braided textile, the fourth ply being a UD nonwoven fabric and a homogeneous plastic outer ply being arranged on the outside of the spring.
Table 6 shows the fiber materials used for exemplary embodiment 3 with their properties, the properties which are known from the prior art and have merely been compiled here.